In general, a field effect transistor (FET) switch functions by receiving a control (or gate) bias which is sufficient to cause the FET channel to conduct in an "on" state and not to conduct in an "off" state. Such switches can be constructed, for example, using metal oxide semiconductor field effect transistors (MOSFETs), junction field effect transistors (JFETs) and metal-semiconductor field effect transistors (MESFETs). In operation, with no gate bias with respect to the source and drain of the FET, the channel of the FET does not conduct as the channel is depleted of carriers. In the case of an n-channel device, for example, this means that there are no excess electrons to facilitate the transport of a current. For an n-channel device, a positive potential with respect to the channel (at source and drain potential) can be applied to the gate, and the channel carriers (e.g., electrons) are enhanced. The channel resistivity is modulated by the magnitude of the difference between the source and drain potential and the controlling gate potential. The greater the difference, the more the conductivity of the channel is enhanced up to a value determined by the doping density of the N-type channel semiconductor.
An FET switch can be used in discrete and integrated circuits as voltage controlled switches either connecting or disconnecting the source from the drain of the FET device. For example, a signal to be switched can be connected to the source, and a fixed bias potential can be placed on the gate to control the connection. (It should be noted that, in a device that is axially symmetrical, the designation of source and drain is arbitrary.) In the case of a time-varying signal, as the voltage magnitude of the signal applied to the source varies, it varies the bias potential difference between the source and the gate. This modulates the resistance of the channel between the source and drain.
Placing a resistive load from the drain to signal ground will cause a current to flow through the FET channel. However, as the channel resistance is modulated by the applied source signal, a wave form that is not an exact duplicate of the applied signal wave form can appear across the load resistor. The result is an output wave form that contains intermodulation products of the input signal. This distortion limits the dynamic range of the signal passing through the FET switch and hence limits the amount of information that can be switched through the FET switch device. Typically, such distortion caused by channel conductivity modulation is greater in metal oxide semiconductor (MOS) devices than in junction devices because the complex nature of the gate oxide produces a high order of non-linearity. In the JFET, the conductivity approximates a squared function of the applied gate-source voltage. However, in the MOSFET, this relation contains higher order non-linearities as evidenced in a Voltera series of its transfer function. This causes even relatively narrow band signals to be distorted by third, fifth and seventh order intermodulation products.
There are conventional methods for decreasing distortion in the output waveform of a switch that involve paralleling N-channel and P-channel devices. For example, a single N-channel FET can be used as a switch, but as the channel to gate voltage decreases with increased input voltage, the channel resistance will increase due to decreased carrier enhancement. A P-channel FET, requiring a negative gate bias to turn the channel "on", displays the same behavior with the resistance increase occurring with an increased negative input voltage. The paralleling of an N-channel and a P-channel device can decrease the resistance curvature about zero volts, and thus decrease the distortion in the output waveform of the switch. This paralleling can result in reducing the slope of the resistance versus input voltage to approximately zero thus minimizing the distortion in the voltage range around zero. However, compensation of the resistance curve over a wide range of input voltages is not possible with such a design.
It would be advantageous to provide a low distortion field effect transistor switch which allows a wide range of signals to be switched while reducing or eliminating the negative effects of intermodulation in the output signal.